This invention relates to systems and methods for writing and reading positioning patterns on data storage systems, and particularly to systems and methods for controlling the writing or reading of precisely positioned reference data and other reference data in sealed disk drive units.
In the present state of the art pertaining to digital storage on movable media, extremely high track densities (of the order of 1000-2000 tracks per inch) have become feasible. Workers in the art have constantly sought to increase the number of usable tracks per disk, i.e. the track density, since it is intuitively recognizable that doubling the number of tracks per inch effectively doubles the amount of stored data. As track densities were increased, it was soon found that open loop systems, such as incremental positioning systems without actual head position feedback, could not meet the accuracy requirements. Such previously minor factors as thermal errors and manufacturing tolerances could not be accepted because of the very small track spacings. Consequently, developers went to embedded servo track systems, and sophisticated dedicated servo surface systems which may also be combined with some embedded servo track information. Embedded servo track systems have prewritten reference patterns among the data tracks which enable precise location of magnetic heads for writing and reading during operation. Dedicated servo surface systems have a single disk surface on which there is only recorded head position information. This information is continuously read in order to position the remaining read/write heads mounted on the common actuator. The constant quest for ever higher track densities has led to modern systems which have 2,000 to 4,000 or more tracks per inch, for magnetic media.
Some of the techniques heretofore used have written the required reference patterns by accurately positioning the head assembly by a separate apparatus which is typically not part of the final drive assembly. For example, an external rotary or linear actuator may be attached to the head assembly drive actuator. With the position of the external actuator being accurately controlled, the head assembly actuator correspondingly follows. This, however, can result in positioning inaccuracies due to various factors such as the friction in the coupling between the two actuators, the relative position of the external actuator and the servo head, and errors between the external position and actual internal head position.
A serious disadvantage is the fact that the disk drive unit must be partially disassembled in order to obtain access to the head assembly actuator, and thereafter maintained in a clean room for the servo track writing operation. A clean room is required to provide the equivalent environment of a clean sealed disk drive assembly. A purified atmosphere without particulate contaminants is needed to "fly" the magnetic head assembly at the necessary close spacing (of the order of a few microinches) to the surface of the disk. Otherwise, particulates, such as ordinary dust, appear as relatively massive objects that affect both the aerodynamics and recording.
A substantially more costly technique, but one which may be adequately precise, mounts a retroreflector assembly on the head assembly actuator, and directs reflected light to a fixed laser interferometer, which can give an extremely precise position signal for the head actuator in the form of a distance measurement to the retroreflector. Then, an external actuator can be used to position the head drive, or the head drive can be placed in a servo loop which uses the interferometer for position feedback This expedient is, however, expensive and as usually practiced involves partial disassembly and clean room operation.
It is evident, therefore, that none of the available expedients provides a suitably low cost, high precision, method for writing position reference information on a disk storage medium, or particularly for doing so with a sealed type of drive and under conditions and with procedures suitable for mass production operations.
There are also other circumstances in which it is desired to sense or control a part of the dynamic interior mechanisms of an open or a sealed disk drive, in terms of determining where the head mechanism is, or the angular position of the disk itself, or both. In some instances in early testing of a unit, for example, it is desired to check data transfer operations at a number of specific track positions. Without a full operating system and servo information this cannot readily be accomplished, if at all, by present techniques. As a different example, the angular position of the rotating disk may be needed, again prior to or independently of, other position information without disassembly of the unit.